1. Field of the Invention
The invention relates to polishing optical surfaces.
2. Description of the Prior Art
A lens, for example an ophthalmic lens, has two opposite optical surfaces connected by an edge surface that is generally inscribed in a circular cylinder.
At present a distinction is drawn between four categories of optical surfaces, namely:
spherical surfaces, which are well known in the art,
aspherical surfaces, which are derived from spherical surfaces,
toric surfaces, and
atoric surfaces, which are derived from toric surfaces
To facilitate an understanding of the following disclosure, one example of the geometrical construction of a toric surface is described next, with reference to FIG. 1.
A torus T, only a portion of which is shown, is obtained by rotation of a circle of radius R2 about an axis Al in the plane of said circle.
The point on the circle at the greatest distance from the axis A1 traces out a circle of radius R1. The radii R1 and R2 are respectively referred to as the larger radius and the smaller radius of the torus T.
In this representation, R1 is much greater than R2.
The circles of radius R1 and R2 are respectively in a plane P1 perpendicular to the axis Al and a plane P2 containing the axis Al, and the planes P1 and P2 intersect along a straight line A2.
A cylinder with axis A2 and of radius R3 (which here is much less than the radius R2) intersects the torus T along a curve C delimiting a toric surface S, which has two plane symmetries: one with respect to the plane P1, and the other with respect to the plane P2.
The intersection of the toric surface S with the plane P1 is a circular arc of radius R1, referred to as the larger meridian M1 of the toric surface S, and the intersection of the toric surface S with the plane P2 is a circular arc of radius R2, referred to as the smaller meridian M2 of the toric surface S.
The larger meridian M1 has a curvature C1 whose value is equal to the reciprocal of the larger radius R1 and the smaller meridian M2 has a curvature C2 whose value is equal to the reciprocal of the smaller radius R2.
Clearly the curvatures of the meridians M1 and M2, which are referred to as the main meridians, are sufficient for a complete definition of the shape of the toric surface S, which is concave in the direction of the axis Al and convex in the opposite direction.
If the toric surface is that of a lens made from a material having a refractive index n, two dioptric powers D1 and D2 for the surface S are defined, on the basis of the curvatures C1 and C2, by the following equations:
D1=(nxe2x88x921)C1, and
D2=(nxe2x88x921)C2.
In the following disclosure, a given surface is considered to be atoric if there is a toric surface which has an offset at any point relative to said atoric surface whose absolute value is less than a chosen value. Here this value is chosen arbitrarily as 0.2 mm for a diameter of 80 mm, but it can be slightly different without departing from the scope of the invention.
At present, optical surfaces have extremely severe constraints on their accuracy, on the one hand with regard to their shape, for which the tolerances are of the order of one micrometer (1 micrometer=10xe2x88x926 meter), and on the other hand with regard to their roughness, for which the tolerances are of the order of one nanometer (1 nanometer=10xe2x88x929 meter).
After roughing out the atoric surface by appropriate machining, the roughness of the roughed out surface is reduced by a polishing step, possibly preceded by a clear polishing step.
The polishing is delicate because it must reduce the roughness of the surface without deforming it.
An optical surface with circular symmetry, such as a spherical surface, can be polished by means of a tool having a polishing surface with a shape complementary to that of the optical surface, the tool and/or the lens being rotated about the axis of symmetry of the optical surface so that the polishing surface rubs against the optical surface.
On the other hand, polishing other types of optical surface gives rise to more problems.
A distinction is drawn between two categories of clear polishing and polishing tools, namely a first category of tools whose diameter is small compared to that of the lens and a second category of tools whose diameter is close to, or possibly greater than, that of the lens. The two categories of tools give rise to totally different clear polishing and polishing techniques, respectively.
Illustrating the first category, the Japanese document JP-09 396 666 discloses a clear polishing tool for an aspherical convex lens and which comprises:
a basic substrate,
an elastic member adhering to the surface of the substrate, and
a surface member adhering to the surface of the elastic member.
The curvature of a spherical surface for the basic substrate, the elastic member and the surface member is identical to a spherical surface of which the working surface of an aspherical surface lens is an approximation.
During the clear polishing process, the lens is rotated and the tool is simultaneously pressed against the working surface.
Because the tool is small compared to the lens, it is necessary to provide a complex kinematic system so that the tool is swept over the whole of the working surface. This proves to be a long and complicated process.
Furthermore, given the relative rotation of the tool and the lens, the tool tends to deform the surface of the lens and impart its own spherical shape to it, at least locally, and the tool is therefore difficult to use on toric or atoric surfaces.
The invention aims to propose a polishing tool and a polishing method using that tool to polish an atoric surface quickly and uniformly whilst at the same time conforming to the constraints on accuracy mentioned above.
Machining mineral glass lenses requires the removal of more material than machining organic glass lenses and causes subsurface microcracks to appear, requiring a longer polishing time to eliminate them, which leads to deformations and inaccuracies in the final shape of the surface of the lens.
The invention is therefore preferably applied to organic glass lenses, which do not have the drawbacks of mineral glass lenses previously cited.
In a first aspect, the present invention proposes a tool for polishing an optical surface of a lens, the tool including:
a rigid support including a support surface,
a first layer called the buffer, made from an elastic material, and covering at least part of the support surface and including:
a first surface adhering to the support surface, and
a second surface opposite the first surface,
a second layer called the polisher, covering at least part of the buffer, and including:
a first surface adhering to the second surface of the buffer, and
a second surface called the polishing surface opposite the first surface and adapted to polish the optical surface of the lens by rubbing against it,
wherein the polishing surface is a toric surface and has two circular main meridians with respective curvatures C1, C2 such that the curvature C1 is much less than the curvature C2, and, to be able to polish an atoric optical surface, the buffer is adapted to be compressed elastically and the polisher is adapted to be deformed to espouse the atoric surface.
During polishing, the tool and the surface to be polished are moved relative to each other with two movements in two perpendicular directions, each of which follows one of the meridians of the polishing surface.
According to other features of the tool:
the buffer has a uniform thickness eT normal to its second surface and the polisher has a uniform thickness eP normal to its polishing surface;
the thickness eT of the buffer is from 4 mm to 6 mm;
the thickness eP of the polisher is from 0.5 mm to 1.1 mm.
In a preferred embodiment the support surface is a toric surface and has two main meridians coplanar with the main meridians of the polishing surface, the meridians having respective curvatures CS1, CS2 satisfying the following equations:             1              C        ⁢                  xe2x80x83                ⁢        S        ⁢                  xe2x80x83                ⁢        1              =                  1                  C          ⁢                      xe2x80x83                    ⁢          1                    -              e        T            -              e        P                        1              C        ⁢                  xe2x80x83                ⁢        S        ⁢                  xe2x80x83                ⁢        2              =                  1                  C          ⁢                      xe2x80x83                    ⁢          2                    -              e        T            -              e        P            
The above specifications enable the tool to be produced as a function of the curvatures C1, C2 to be imparted to the polishing surface and the thicknesses eT and eP of the buffer and the polisher.
In accordance with other features, more specifically concerning the production of the buffer:
the buffer is made of a material which is deformed by more than 5% by a pressure of 0.04 MPa;
the buffer is made of elastomeric material or polyurethane foam.
The polisher can be made of woven fabric, felt, or preferably of polyurethane foam.
The tool that has just been described is used to polish an atoric optical surface of a lens such as an ophthalmic lens, preferably a lens made of organic glass.
The lens having a circular edge surface having a given diameter, the tool preferably has a circular section whose diameter is greater than the diameter of the edge surface of the lens.
In another aspect, the invention proposes a method of polishing an atoric optical surface of an ophthalmic lens corresponding to a given prescription, the method including the following steps:
taking into account characteristic geometrical values of the optical surface of the lens, and
using a tool as defined above, during use of which tool the polishing surface of the polisher and the optical surface of the lens are in relative bearing and rubbing interengagement.
According to the invention, the above method includes, prior to the step of using the tool, a step of determining the tool, the tool determination step comprising the following sub-steps:
a) determining a toric surface close to the optical surface of the lens, the toric surface, called the best torus, comprising two circular main meridians having respective curvatures C*1, C*2 such that the curvature C*1 is much less than the curvature C*2,
b) determining a toric surface corresponding to the given prescription, the toric surface, called the reference torus, comprising two circular main meridians having respective curvatures Cxe2x80x21, Cxe2x80x22 such that the curvature Cxe2x80x21 is much less than the curvature Cxe2x80x22,
c) determining respective values of the curvatures C1, C2 of the polishing surface from the following equations:
C1=C*1+xcex94C1, and
C2=C*2+xcex94C2,
in which:
xcex94C1, called the first correction, is a function of:
the curvatures C*1, C*2 of the best torus,
the curvatures Cxe2x80x21, Cxe2x80x22 of the reference torus, and
the diameter of the edge surface of the lens, and
xcex94C2, called the second correction, is constant
In step c), the first correction xcex94C1 is, for example, an affine function of:
the difference C*2xe2x88x92C*1 between the curvatures C*2, C*1 of the best torus, and/or
the difference Cxe2x80x22xe2x88x92Cxe2x80x21 between the curvatures Cxe2x80x22, Cxe2x80x21 of the reference torus.
In one embodiment, in step c), the value of the first correction xcex94C1, is expressed in mxe2x88x921, is given by the following equation:
xcex94C1=a+b(Cxe2x80x22xe2x88x92Cxe2x80x21)+c[(Cxe2x80x22xe2x88x92Cxe2x80x21)xe2x88x92(C*2xe2x88x92C*1)]+d."PHgr"2,
where a, b, c, d are parameters of constant value and "PHgr"2 is the diameter of the edge surface of the lens.
The parameters a, b, c, d are defined as follows, for example:
the value of the parameter a is from 0 to 4 mxe2x88x921, preferably from 0.2 mxe2x88x921 to 3.4 m1.
the value of the parameter b, with no units, is from 0.01 to 0.3, preferably from 0.05 to 0.25.
the value of the parameter c, also with no units, is from xe2x88x922 to xe2x88x920.01, preferably from xe2x88x921.5 to xe2x88x920.1.
the value of the parameter d is from xe2x88x92100 mxe2x88x922 to 0, preferably from xe2x88x9260 mxe2x88x922 to xe2x88x922 mxe2x88x922, the diameter of the edge surface of the lens being expressed in m.
The second correction xcex94C2 is, for example, from 0 to 0.8 mxe2x88x921, preferably from 0.1 mxe2x88x921 to 0.64 mxe2x88x921, for example equal to 0.37 mxe2x88x921.
In step a), the determination of the best torus is preferably carried out by the mathematical method known as the least squares method.
In one embodiment, in step a), the determination of the best torus is carried out for only a portion of the atoric surface of the lens, the portion having a circular circumference coaxial with the edge surface of the lens.
In another aspect, the invention proposes a unit for determining the tool for application of a method as just described, including:
a computer including:
means for computing the curvatures C*1, C*2 of the best torus as a function of characteristics of the optical surface of the lens,
means for computing the curvatures Cxe2x80x21, Cxe2x80x22 of the reference torus as a function of the prescription, and
means for computing curvatures C1, C2 of the polishing surface as a function of the curvatures C*1, C*2, Cxe2x80x21, Cxe2x80x22 and the diameter of the edge surface of the lens,
an input device connected to the computer and including means for entering characteristics of the optical surface of the lens,
a memory connected to the computer and including:
a first memory area for storing geometrical characteristics of the atoric surface of the lens,
a second memory area for storing the curvatures C*1, C*2 of the best torus,
a third memory area for storing the curvatures Cxe2x80x21, Cxe2x80x22 of the reference torus, and
a fourth memory area for storing the curvatures C1, C2 of the polishing surface, and
an output device connected to the computer and including means for displaying at least values input to the computer.
In a further aspect, the invention provides an installation for polishing ophthalmic lenses and suitable for implementing a method as just described, the installation including:
a lens support,
a tool-holder,
means for creating relative movement of the lens support and the tool-holder, and
a digital control unit including a tool determination unit as described above.
The invention can be used to polish an atoric optical surface quickly and efficiently without deforming it. The compressible buffer assures permanent contact between the polisher and the atoric surface of the lens.
Other objects and advantages of the invention will become apparent in the light of the following description, which is given with reference to the accompanying drawings.